We report follow-up studies of 22 cases of the Norrbottnian type of Gaucher disease ("type III"). The series was divided into 2 main groups of families depending on their birth province (Norrbotten, Västerbotten). The distribution and types of organ manifestations and complications were the same in both groups, each of which was considered to be genotypically homogeneous. The severity of the clinical symptoms and signs and the course of the disease differed markedly not only between families but also between siblings. Splenectomy accelerated deterioration, particularly with regard to skeletal and central nervous system manifestations. On a clinical basis it is concluded that the Norrbottnian type of Gaucher disease, which has now been diagnosed in about 40 cases, is probably due to a unique mutation which may have happened several hundred of years ago in northern Sweden.